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SIL (Safety Integrity Level)

SIL – Unit of Measurement for Risk Reduction

08/05/2011 | Autor / Editor: Dr. Andreas Hildebrandt / Jörg Kempf

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In principle, the term “SIL” is a unit of measurement that can be used to quantify risk reduction. As is usual with units of measurement, the required degree of risk reduction is specified first of all (risk analysis).

Then protective equipment (Z function) is installed to reduce the original risk to the required degree. A final step is required to prove that the objective has been reached. This implies that evaluation criteria are defined to establish the degree of risk reduction achieved by the protective equipment. Essentially, the following points apply here:

  • Preventing systematic faults using an FSM system [Fig. 5, left side]
  • Avoiding faults through redundancy and/or fail-safe design [Fig. 4, tables]
  • Detecting faults using diagnostics [Fig. 5, tables]
  • Assessing the probability of failure caused by random faults [Fig. 5, formulas]

The structure and the probability of failure are essential for the SIL of the protective equipment, whereby all activities and results have to be performed, verified and documented using the FSM system. The FSM system is the vital element in reaching a specific SIL. The PFD calculation is another vital element for reaching the SIL. However, past studies show that the failure of protective equipment can almost always be traced back to systematic faults and is only caused by random faults in a few rare cases [5] [6]. The FSM system should therefore be the main focus and not the PFD calculation.


[1] EN 61508-4, “Functional safety of safety-related electric/electronic/programmable electronic systems, Part 4: terms and abbreviations”, sections 3.6.5 and 3.6.6, November 2002

[2] Dirk Hablawetz, Norbert Matalla and Gerhard Adam, “IEC 61511 in der Praxis - Erfahrungen eines Anlagenbetreibers” (IEC 61511 in practice - Experiences of a plant operator), atp 10/2007

[3] EN 61511; “Functional safety - Safety instrumented systems for the process industry sector - Parts 1–3”, May 2005

[4] VDI/VDE 2180, “Safeguarding of industrial process plants by means of process control engineering (PLT)”, 2009

[5] HSE, “Out of control – Why control systems go wrong and how to prevent failure”, HSE Books, ISBN 0-7176-2192-8

[6] European Commission Environment

* Dr. Andreas Hildebrandt is Team Leader Training / Committee Work, Division Process Automation with Pepperl+Fuchs, Germany


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